Injection molding is a common technique for manufacturing small plastic structures in large quantity at low cost. When such structures comprise smoothly curved surfaces such as lens-like elements with maximum convexity or concavity less than or equal to a few millimeters, mold manufacturers generally use mold inserts, which are steel rods with diameters slightly larger than that of the lens-like structure, having a diamond-turned surface at the working end of the insert that faithfully reproduces the desired convex or concave lens-like surface shape. These inserts are then placed in machined holes in the molds, becoming an integral part thereof, with the working surface of the mold generally continuous with the working surface of the mold insert. It is common that the company making molds for injection molding applications contracts with a diamond turning company to provide the inserts. A first disadvantage of this technology is that the diamond turning company may take a considerable amount of time to fabricate the inserts required for a mold. On many occasions, after the initial delivery of the mold and testing of plastic integrated optical structures made therefrom, the mold fabricators' customer may require that the mold inserts be modified, in an attempt to secure the best optical performance of the plastic structure, which incurs further long delays. This long turn-around time greatly increases product development time and may be unacceptable in a competitive marketing situation. A second disadvantage of this technology is the high cost of fabricating the diamond turned inserts, which increases the costs of the mold, and ultimately, the cost of the plastic integrated optical structure. A third disadvantage of this technology is that the conventional diamond turning process itself is limited in the surface shapes that it can create: All such surfaces must have rotational symmetry. In fact, a common class of lens shapes, the cylindrical lens, does not conform to this requirement. Another common lens shape that does not have rotational symmetry is the toric lens, which in a larger scale is commonly used as an eyeglass lens for the correction of astigmatism. This lens exhibits two peaks and two valleys per rotation of the lens, and may be considered to be a combination of a rotationally symmetric spherical lens and a non-rotationally symmetric cylinder lens. This lens shape also cannot be fabricated using conventional diamond turning. The present invention solves one or more of the above discussed disadvantages.